US officials said 59 Tomahawk cruise missiles were fired from Navy ships in the Mediterranean at the Shayrat airfield at 3:40 am (0040 GMT) (AFP Photo/Ford WILLIAMS)

US forces fired a barrage of cruise missiles at a Syrian airbase Friday in response to what President Donald Trump called a "barbaric" chemical attack he blamed on the Damascus regime.

The massive strike -- the first direct US action against President Bashar al-Assad's government and Trump's biggest military decision since taking office -- marked a dramatic escalation in American involvement in Syria's six-year civil war.

It followed days of outrage at images of dead children and victims suffering convulsions from the suspected sarin gas attack in the rebel-held town of Khan Sheikhun.

US officials said 59 Tomahawk cruise missiles were fired from Navy ships in the Mediterranean at the Shayrat airfield at 3:40 am (0040 GMT), targeting the base from where Washington believes Tuesday's deadly attack was launched.

Calling the strike a "flagrant aggression", the Syrian army said it had killed six people and caused extensive damage to the base.

The attack was hailed by the Syrian opposition and supported by US allies including Britain, France, Israel, Saudi Arabia and Turkey.

But it was denounced by Assad allies Iran and Russia, with Moscow warning that it would inflict "considerable damage" on US-Russia ties and halting an agreement with Washington aimed at avoiding clashes in Syrian airspace.

Trump announced the strike in a brief televised address delivered hours after the UN Security Council failed to agree on a probe into the suspected chemical attack.

Syria's military has denied ever using chemical weapons, but Trump accused Assad of a "very barbaric attack" in which "even beautiful babies were cruelly murdered."

At least 86 people, including 27 children, were killed in Khan Sheikhun and more than 500 wounded.

"Tonight I call on all civilised nations to join us in seeking to end this slaughter and bloodshed in Syria and also to end terrorism of all kinds and all types," Trump said.

The missiles were fired from the USS Porter and the USS Ross, which belong to the US Navy's Sixth Fleet and are in the eastern Mediterranean.

The strike targeted radars, aircraft, air defence systems and other logistical components at the base south of Homs in central Syria.

US officials said measures had been put in place to avoid hitting sarin gas they said was stored at the airfield.

"The airbase was almost completely destroyed -- the runway, the fuel tanks and the air defences were all blown to pieces," the Syrian Observatory for Human Rights said.

The Britain-based monitoring group said at least seven servicemen were killed, including an air commodore.

The base was the second most important for Syria's air force, Observatory chief Rami Abdel Rahman told AFP, after the Latakia airbase in Assad's coastal heartland where Russia also maintains extensive facilities.

In a statement read on state television, the Syrian army confirmed the strike and gave the toll of six dead, without specifying whether they were civilians or military personnel.

It accused Washington of being allied with jihadists like the Islamic State group, and said it was seeking to "justify this aggression" by pointing the finger at Damascus for the suspected chemical attack "without knowing the truth."

Syria's opposition and rebel fighters, who have for years urged more direct US military action in support of their uprising, hailed the strike and called for more.

The National Coalition, the main opposition grouping, called on Washington to take further steps to "neutralise" the regime's air power.

"We hope for more strikes... and that these are just the beginning," spokesman Ahmad Ramadan told AFP.

The White House was quick to paint the decision as limited to deterring the use of chemical weapons, and not part of a broader military campaign to remove Assad by force.

"The intent was to deter the regime from doing this again, and it is certainly our hope that this has had that effect," Pentagon spokesman Navy Captain Jeff Davis told reporters.

Moscow, which launched a military intervention in support of Assad's forces in 2015, had warned of "negative consequences" if Washington carried out military action in Syria.

Russia stood by Damascus this week despite the global uproar, insisting that the chemical weapons that caused the deaths in Khan Sheikhun had been stockpiled by "terrorists" on the ground and possibly released by a conventional strike.

It demanded an emergency meeting of the UN Security Council to discuss the US strike, calling it a "gross... violation of international law."

The Kremlin said President Vladimir Putin considered it "aggression against a sovereign state" and that it would inflict "considerable damage" on US-Russia ties.

Moscow also announced it was suspending a deal with the United States that was aimed at preventing incidents in Syrian airspace, where Russian warplanes and aircraft from a US-led anti-jihadist coalition are both operating.

US officials said Russia's military in Syria was informed of the strike beforehand in order to avoid casualties that could prompt a broader crisis.

Just days before he is due to visit Moscow, US Secretary of State Rex Tillerson accused Russia of being incompetent or complicit in permitting Assad's actions.

Tillerson said the attack should leave no one in any doubt that Trump is willing to act if any actor "crosses the line."

It will send ripples around the world, from Pyongyang to Tehran, as nations and leaders take the measure of the novice but often bellicose president.

The timing of the strike, during a meeting with China's President Xi Jinping, will give weight to Trump's threats to deal with North Korea's nuclear and missile programmes unilaterally if necessary.

On Wednesday, Trump had decried the suspected chemical attack as an "affront to humanity."

"It crossed a lot of lines for me," Trump said, alluding to Barack Obama's failure to enforce his own "red line" on the use of chemical weapons in Syria four years ago.

In 2013, Trump had urged then-president Obama not to intervene against Assad.

The Khan Sheikhun incident appears to have marked a turning point for Trump, just days after his administration signalled it was no longer seeking the Syrian leader's departure from power.

Tillerson called Thursday for "a political process that would lead to Assad leaving" and said his future role in the country was "uncertain."